1. Field of the Invention
The present invention relates to an optical amplification apparatus for amplifying signal light using Raman amplification, and in particular to an optical amplification apparatus having a function for detecting an input interruption of signal light.
2. Description of the Related Art
Recently, the development of techniques has been progressed for achieving for example an expansion of optical amplification bands, or a reduction in repeater loss in various types of optical transmission systems, through the construction of optical amplification apparatus making use of Raman amplification. For example, an optical amplification apparatus is proposed with a construction as shown in FIG. 10, where a Raman amplifier is disposed prior to for example an erbium doped optical fiber amplifier (EDFA), and Raman amplified signal light is input to the EDFA. Furthermore, in the future, it may be considered that a Raman amplifier alone constructs the optical amplification apparatus.
However, with a general optical transmission system which repeatedly transmits signal light using an optical amplification apparatus, for example in the case where the signal light is cut off due to the occurrence of an open circuit of the transmission path or a disconnection of a connector, it is necessary to instantly detect the input interruption of the signal light in the optical amplification apparatus. The reason why such input interruption detection is necessary is to avoid for example; a problem where, since an AGC for controlling an amplification gain of signal light to be constant, or an ALC for controlling the level of output light to be constant, are generally applied to an optical amplification apparatus, in the case of an input interruption in the signal light, an amplification operation is controlled so as to obtain a predetermined output light by only a noise component which is generated in the optical amplification apparatus, or a problem where when the input interruption of the signal light is recovered in such a condition, in the case where an EDFA is used as the optical amplification apparatus, a surge is generated bringing damage to the apparatus.
With the optical amplification apparatus using an EDFA, a so-called shutdown control has been performed which detects an input interruption of the signal light and shuts off supply of excitation light to the erbium doped fiber (EDF). More specifically, for example as shown in FIG. 11, when wavelength division multiplexed (WDM) signal light sent from a prior stage EDFA (not shown in the figure) via a transmission path is to be collectively amplified by an EDFA, a part of the WDM signal light input to the EDFA is branched by an optical coupler, and the power of the branched light is monitored by an light power monitor section. The light power monitored by the light power monitor section becomes, for example as shown in FIG. 12A, the light power corresponding to the sum of a signal light component contained in the WDM signal light and amplified spontaneous emission light (ASE light) which is generated and accumulated in the prior stage EDFA and so forth.
With such a construction, if an input interruption of the WDM signal light occurs due for example to an open circuit of the transmission path connected to the prior stage EDFA, a disconnection of a connector, or the like, then as shown in FIG. 12B, the light power monitored by the aforementioned light power monitor, becomes approximately zero. Consequently, with the shutdown control in the conventional EDFA, in the case where the light power monitored by the light power monitor section falls to a predetermined threshold value or below, the EDFA control section judges an input interruption of the WDM signal light to perform a control to shut down the supply of the excitation light to the EDFA.
In the case where the above described conventional EDFA shutdown control is applied to an optical amplification apparatus as shown in the aforementioned FIG. 10 where a Raman amplifier and an EDFA are combined, then caused by the generation of noise light due to Raman amplification, there is a problem that it is difficult to accurately judge an input interruption of the signal light. This noise light due to Raman amplification, is noise light which is also generated in the case where, in a situation where the signal light is not input, Raman excitation light only is emitted into an amplifying medium, and in general is referred to as Raman scattering light due to pumping light. Here, in contrast to the amplified spontaneous emission (ASE) light generated in the EDFA, the abovementioned noise light generated in the Raman amplifier is referred to as amplified spontaneous Raman scattering (ASS) light.
In an optical amplification apparatus where a Raman amplifier and an EDFA are combined, the power of the input light to the EDFA, to be monitored by the light power monitor is, for example as shown in FIG. 13A, specifically the light power corresponding to the sum of the signal light component, the ASE light component which is generated and accumulated in the prior stage EDFA and the like, and the ASS light component generated due to Raman amplification of the own stage Raman amplifier. Then, when an input interruption of the signal light occurs, the light power monitored by the aforementioned light power monitor, becomes as shown in FIG. 13B, the light power corresponding to the ASS light component. Consequently, in order to perform a positive shutdown control for such an optical amplification apparatus unit, it becomes a subject to perform the correction in accordance with the aforementioned ASS light component; for the threshold value being the reference for judging an input interruption in the shutdown control in the conventional EDFA.
Furthermore, with the optical amplification apparatus which uses Raman amplification, since extremely high level excitation light is emitted into the optical fiber which constitutes the transmission path, there is the possibility that due to an open circuit of the transmission path or a disconnection of the connector, the excitation light may be emitted to the outside. In such a case, it is desirable to take measures such as, immediately lowering the excitation light power to a safe level, or switching off the drive condition of the excitation light source. However, to optical amplification apparatuses using Raman amplification, which have been proposed up to this date, the abovementioned measures have not been specifically applied.
The present invention addresses the above mentioned points, with the object of providing an optical amplification apparatus which uses Raman amplification and which can reliably judge an input interruption of signal light, and providing an optical amplification apparatus which can shut down the supply of excitation light in accordance with a judged input interruption of the signal light.